EXPERIMENTAL INVESTIGATION ON PNEUMATIC COMPONENTS

Transcription

1 Conference on Moelling Flui Flow (CMFF 03) The 12 th International Conference on Flui Flow Technologies Buapest, Hungary, September 3-6, 2003 EXPERIMENTAL INVESTIGATION ON PNEUMATIC COMPONENTS Zoltán MÓZER, stuent Ákos TAJTI, stuent Viktor SZENTE, assistant lecturer Department of Flui Mechanics Buapest University of Technology an Economics Bertalan Lajos u. 4 6., H-1111 Buapest, Hungary Tel.: (+36 1) , Fax: (+36 1) ABSTRACT Pneumatic Electric Braking Systems (EBS) are frequently use in commercial vehicles. These systems are controlle at about 100 Hz, thus the ynamic behaviour of them is essential to know. Since solenoi valves are playing important role in control, the knowlege on the ynamic behaviour of these valves is essential. This paper presents the evelopment of a computer-controlle experimental facility, for investigation of the C q flow coefficient. This ocumentation presents the comparison of the experimental methos to international literature an numerical flow simulations. Key Wors: pneumatic control, flow coefficient, solenoi valve NOMENCLATURE m [kg] mass of gas in the chamber p [Pa] absolute pressure A [m 2 ] orifice cross-section C m [-] orifice mass flow parameter C q [ K /( m / s) ] orifice flow coefficient Q m [kg/s] mass flow rate R [J/kg/ºK] perfect gas constant T [ºK] temperature V [m 3 ] chamber volume κ [-] specific heat ratio Subscripts an Superscripts cr values at critical pressure ratio ownstream values u upstream values 1. INTRODUCTION From the beginnings of the 90 s, companies starte to evelop an manufacture intelligent electro-pneumatic braking systems [1]. In these evices the proceure of sensing an execution is one by electro-pneumatic elements, like sensors an Electric Braking System (EBS) moulators. The electric control makes the braking process much faster, an provies safety-enhancing methos, like the ABS (Antilock Braking System) function [2, 3]. The EBS moulator is an essential part of a moern electro-pneumatic braking system, which ensures the controlle pressure in the brake chamber, in all conitions. This way it can provie continuous connection between the wheels an the roa surface, increasing safety [4]. Electro-pneumatic magnetic valves execute the control itself, insie the moulator. The valves are controlle by electric signals from the control equipment. These valves create fast, impulse-like flow between the volumes at ifferent pressure with less than 0.01s perioic times. Thus, the electroynamic features of the magnetic valves (openclose time), geometrical an hyroynamic (flow rate) properties, influence the operation of the whole pneumatic system. There are two main ways of investigation of ynamic behaviour of the magnetic valves: Experimental investigations Numerical simulations

2 By experimental investigations the valiity of the numerical moel coul be verifie. The comparison between the measure an the simulate solution can be mae easily, an if the ifference between the results is small enough, the numerical moel can be use for further research an evelopment, reucing the cost of the expensive an time-consuming measurements. In case of experimental investigations, a lot of characteristic values have to be consiere at the same time, e.g. the pressure ratio, the temperature before the valves an the flow rate through the valves, which makes the measurement more complicate. In orer to make the research an evelopment projects simpler with more accurate results, a computer controlle measuring evice has been built recently in the laboratory of the Department of Flui Mechanics. The bench is fully automate, which uses a computer-controlle ata acquisition evice. The avantages of this sophisticate bench are that the recoring of the measure ata is quick an accurate, an the PC support provies easy reloa an analysis of the collecte ata. This paper introuces the bench with a nonstanar from-chamber-to-chamber measurement setup, an the principles of the pneumatic features of the magnetic valves. The suitability of the evice for ynamic measurements will be interprete through case stuies. With the experimental measurements, the flow coefficient C q which characterizes the valve pneumatically will be etermine. As an earlier work a 3D moel of the valve has been create using FLUENT CFD (Computational Flui Dynamics) software. Another value of flow coefficient can be calculate with the help of CFD simulation, an the results of this metho can be easily compare to the measurement ata. An important goal of the project was to emonstrate that the results coming from ifferent ways are quite similar, an this fact allows using the CFD moel in further research an evelopment projects. The big avantage of this is that the expensive an time consuming measurements can be avoie. 2. THE THEORY OF THE ORIFICE FLOW In pneumatic energy transferring systems, the control an the energy transfer is carrie out by pressurize gas. A number of elements are working in choke flow circumstances. The main feature of choke flows is that there are places where the velocity of the flow reaches the velocity of soun (sonic velocity) as the upstream pressure (p u ) is much bigger compare to the ownstream pressure (p ). In that case the mass flow rate of the gas is proportional to the upstream pressure an inepenent from the ownstream pressure Ieal case During the measurement the flow is stationary. As ieal case, it is assume that in the throttling port the flow is invisci an heat isolate. In this case, accoring to the stanar, the mass flow rate Q m of the gas flowing through the throttling port can be calculate with the following expression [5]: Q p u m = A Cm (1) Tu where A is the orifice cross-section, p u an T u are the upstream pressure an temperature, respectively, an C m is the mass flow parameter. Sonic velocity appears uner the critical pressure ratio. This ratio can be calculate using Eq. (2) in the case of air (κ=1.4): κ κ 1 p 2 = = pu κ + 1 cr (2) where p is the ownstream pressure an κ is the specific heat ratio of the gas. p p If the flow is sonic, i.e. then pu pu cr the mass flow parameter C m is constant: C m 2 κ 2 = R ( κ + 1) κ κ 1 (3) where R is the perfect gas constant. p p If the flow is subsonic, i.e. > then pu pu cr the mass flow parameter C m is a function of the pressure ratio p /p u : C m 2 γ + 1 κ κ p 2 κ p = R ( κ + 1) pu pu (4)

3 2.2. Consiering the viscosity with the flow rate In the ieal case the Eqs. (1), (3), (4) can be use to calculate the mass flow rate, but the real value of the mass flow is ifferent. The reason of this is the contraction of the gas jet, which occurs in the orifice ue to viscosity. By the multiplication of the calculate ieal mass flow an the C q <1 flow coefficient, the effect of the viscosity can be taken into account empirically: p Q = A C C (5) u m m q Tu Accoring to the international stuies an reports [6-9], the flow coefficient can be approximate by the function of the p /p u pressure ratio, by e.g. the Perry moel. (See Chapter 5.1). There are some other resources though, which suggest using constant C q coefficient in a given situation [10-12]. In the actual phase of the research, the main goal is to investigate the C q flow coefficient in experimental way, then compare it to the Perrymoel, an the calculate value of the numerical simulation. After the rearrangement of Eq. (5) the following equation can be obtaine: C q Qm = AC m T u p u (6) As it is shown in Eq. (6) above, the measurement of upstream pressure p u, upstream temperature T u, an the mass flow rate Q m require at the same time to etermine the value of the flow coefficient C q. The value of the mass flow parameter C m epens only on the gas properties, while the orifice crosssection A is a given geometric property of the valve. 3. EXPERIMENTAL TECHNIQUE In this chapter the from-chamber-to-chamber measurement setup will be introuce. The assembly rawing of this measurement can be seen in Figure 1. Figure 1. The from-chamber-to-chamber measurement Chart of symbols: A Compresse gas vessel B Upstream chamber C Downstream chamber D Measure valve E Temperature transmitters F Pressure transmitters G Air filter an pressure regulator H Data acquisition PC I Cut-off valve J Cut-off valve 3.1. Introuction of the from-chamber-tochamber measurement The main purpose of this measurement is to etermine the C q flow coefficient of the investigate pneumatic component. This metho etermines C q from the pressure curve of the ownstream chamber (C). The starting slope of this curve is in close connection with the flow coefficient. The main reason of using this technique was to prouce a measurement metho which can be use even in actual pneumatic systems. Nowaays it is quite easy to integrate pressure- an temperature transmitters into existing pneumatic systems without compromising flow transmission characteristics. In the case of a stanar ISO 6358 measurement [13], a flow meter woul also be necessary, but they are either restrict the flow (e.g. rotameters) or too bulky to be use in highly integrate pneumatic components such as vehicle brake systems. Moreover, the stanar emans strict geometry requirements, which are practically impossible to implement into real-life systems. Using chamber-tochamber measuring methos, on the other han, it is

4 possible to measure even real-life systems without sacrificing functionality. This means that a computerize control system can be ae to existing systems quite easily, enhancing the control an/or the functionality, an it even enables esigners to make irect comparisons between numerical simulations an actual systems. Since this type of measurement functionalities have alreay been installe in moern electro-pneumatic braking systems, it was a straightforwar ecision to prefer this type of measurements against others. To prepare a stanar ISO 6358 measurement an compare it with the results presente in this paper is a subject of future work. The neee components for the measurement are the following: Compresse air vessel (A). This will supply the system with compresse air. The air filter an pressure regulator (G) is an important part of the test bench. First of all the compresse air must be clean an ry. Furthermore, the measurements ha to be one at ifferent pressures. With the help of this component the pressure of the compresse air can be easily set up between each measurement. The next component is the upstream chamber (B). Before the measurement this will be fille up with compresse air to a pressure set by the pressure regulator. After the setup, the chamber is close by the cut-off valve (I). This is neee because the pressure change in the chamber has to be measure. The last component of the system is the ownstream chamber (C). The goal of the measurement is to etermine the pressure-curve in this chamber uring the fill-up process, because the slope of this curve is in close connection with the C q flow coefficient. An example of a measure pressure-curve can be seen in Figure 2. Chamber1 is the pressure-curve of the ownstream chamber, while Chamber2 is the pressure-curve of the upstream chamber. There is a cut-off valve (J) on the chamber for ecreasing the pressure to atmospheric after the measurement. Each chamber has a pressure transmitter (F) an a temperature transmitter (E). The signals of the transmitters are measure with the ata acquisition PC (H). Figure 2. Pressure curve example 3.2. Process of the measurement The compresse air vessel is fille up to about 9 bar relative pressure. The upstream chamber is fille up through the pressure regulator. After this the cut-off valve is close. The pressure of the ownstream chamber is atmospheric an the cut-off valve (J) is close. The investigate valve is close as well. The process starts as the measurement software opens the pneumatic valve, so the pressure of the two chambers starts to equalize. During this the software collects the ata from the transmitters. 4. THE INVESTIGATED VALVE Electro-pneumatic valves are wiely use in many fiels of the inustry. The simplifie sketch of the investigate valve is shown in Figure 3. The valve boy is manufacture with elastomer sealing an closing surface. In the initial state of the coil the valve boy is kept in close state by the return spring. The electromagnetic force which appears after applying voltage to the solenoi isplaces the valve boy an makes the flow possible [14]. Since this paper concentrates on the flow properties of the valve, an the time neee for the opening of the valve is smaller by several orers of magnitue than the measurement process (about 5 ms valve opening time against 5-10 s measurement process time), the ynamics of the valve boy isplacement an the electromagnetic properties have not been taken into account (i.e. it has been assume that the valve opens in an infinitely short time an is always fully opene uring the fill-up process). The main goal of the measurement evice is the experimental investigation of the ynamic behaviour of such electro-pneumatic valves. The solenoi is triggere by irect current which

5 controlle by the computer. The orifice of the investigate magnetic valve was changeable, so the measurements were performe on two ifferent geometries. The values of the orifice iameters are 1.5 mm an 2 mm. Figure 3. The electro-pneumatic valve 5. COMPARISON OF THE RESULTS In this chapter the results of the measurements an the evaluation will be introuce. The results will be compare to simulation results an to international literature Introuction of the Perry-moel For estimation of the C q flow coefficient the Perry-moel can be use as well, which approximates the C q parameter accoring to the international literature, in a polynomial function of the p /p u pressure ratio, for a circular-shape sharpege orifice, as it can be seen in Eq. (7) Introuction of the FLUENT moel As an earlier work a 3D moel of the valve has been create using the FLUENT coe [15, 16]. Another value of flow coefficient can be calculate with the help of CFD simulation, an the results of this metho can be easily compare to the measurement ata. Because of axial symmetry, the 3D valve moel has been transforme to quasi-3d (Q3D) axisymmetric omain. Figure 4. shows the 3D layout of the valve, while Figure 5. shows the Q3D scheme that has been use in CFD simulation. Because of the limitations in the 3D simulation software available, the movement of the valve boy has not been incorporate into the moel [17]. Figure 4. The 3D layout of the valve The simulation software compute the mass flow rate for the initial state, the cross-section of the flow has been given. p u, p an T u have been given as bounary conitions. The value of C q has been euce from the CFD simulation using Eq. (6) an has been compare to the experimental results an the Perry moel. Figure 5. Q3D scheme C q p p p p p = pu pu pu pu pu (7)

6 5.3. Evaluation of the measure ata At the measurement the starting slope of the pressure curve has to be etermine in the ownstream chamber. The initial pressure of the upstream an ownstream chambers will be use for the efinition of the pressure ratio (p /p u ). Then the initial slope of the curve has to be efine, by fitting a straight line to the initial section of the curve (0.1 s from the opening of the valve). From this the mass flow rate can be calculate, therefore it is possible to etermine the C q flow coefficient accoring to Chapter Calculating the mass flow rate As ieal case, it is assume that the gas properties in the ownstream chamber can be calculate using the ieal gas relation: p V = m R T (8) where p an T are the ownstream pressure an temperature, respectively, V is the volume of the ownstream chamber, m is the mass of gas in the ownstream chamber an R is the perfect gas constant. The initial slope of the pressure curve can be calculate by ifferentiating Eq. (8): T V RT Q + m R p = V p m t t t (9) where Q m is the mass flow into the ownstream chamber an is the time erivative of a given t variable. The volume of the chambers are fixe, therefore V the volume variation term p equals zero. t An furthermore, accoring to the measurement T results, the temperature variation term m R t is at least three orers of magnitue smaller than the mass flow term R T Qm uring the initial section of the process (0.1 s from the opening of the valve), so it has eeme negligible. After applying these simplifications an assumptions to Eq. (9), the mass flow rate can be calculate using the following equation: Q m p V = t R T (10) The volume of the ownstream chamber is known, the temperatures in both chambers have p been measure, an the term is the initial t slope of the pressure curves. This means that the mass flow rate can be calculate using Eq. (10), then C q can be obtaine from Eq. (6) Results of the measurements The measurement series were complete in several steps, from 1.2 bar to 10 bar absolute pressure, on two ifferent valve geometries. Figure 6. shows the result of the 1.5 mm orifice, while Figure 7. the result of the 2 mm orifice, both compare to the Perry moel an the simulate values. C q Perry Fluent measurement p /p u Figure 6. The results of the 1.5 mm orifice C q Perry Fluent measurement p /p u Figure 7. The results of the 2 mm orifice

7 6. EVALUATION OF THE RESULTS The iagrams show the results of the Fluent simulation an the Perry-polynomial besie the measure values. It can be seen that the shape of the curves are quite similar. The causes of the ifferences can be the following: The Perry-polynomial is for circular-shape sharp-ege orifices, but the geometry of the investigate electro-pneumatic valve is ifferent. The air flow in the valve has an about 90 change of irection. These are the main causes of the ifferences between the Perry-polynomial an the other two results. The comparison of the measurement an the simulation results gives another ifference, but this is significant only over the critical pressure ratio range ( ). The C q flow coefficient is in close connection with the starting slope of the pressure curve of the ownstream chamber. At the etermination of the flow coefficient the task is to efine the value of the starting slope. Uner the critical pressure ratio, where the flow is sonic, the change of pressure is linear, so the etermination of the starting slope is quite accurate. Over the critical pressure ratio, where the flow is subsonic, the change of pressure is exponential. In this case the etermination of C q is more ifficult, therefore the error can be larger than in the previous case. This can be the main reason of the ifference between the measurement an the simulation over the critical pressure ratio. The comparison of the two measurements orifice with 1.5 mm an 2 mm gives the next ifference. The iagrams show, that the C q of the 1.5 mm orifice is smaller. The reason coul be the following: The 1.5 mm orifice ha been manufacture with ifferent technology. First of all, the surface roughness of the orifice is larger. Furthermore, the inlet shapes of the two orifices are not the same. This means the input of the 1.5 mm orifice has a quite sharp ege at the inlet, while the 2 mm orifice has a minor fillet. This can be seen in Figure 8a. for the 1.5 mm orifice an in Figure 8b. for the 2 mm orifice (the imensions of the fillet are exaggerate for better visibility). The air flow has an about 90 change of irection at the inlet of the orifice. Without the fillet, the flow fills out a smaller part of the crosssection, the contraction will be larger an so the value of C q will smaller too. That is probably the main the reason of the ifference. 1.5 mm 2 mm Figure 8a. The inlet of the 1.5 mm orifice Figure 8b. The inlet of the 2 mm orifice The isplaye curves show that the slopes are quite similar, so the measurements were most likely correct. The error of the 1.5 mm measurement is about 10%, because of the mentione problems, but the error of the 2 mm measurement is only about 5%, which is acceptable. The ifference between the measurements an the simulation results are small enough, so it can be conclue that the simulation can be use in further investigations. 7. SUMMARY In this paper a pneumatic measurement system has been introuce, with a non-stanar fromchamber-to-chamber setup. A number of test measurements were accomplishe on the investigate electro-pneumatic solenoi valve in orer to obtain the C q flow coefficient for ifferent geometries an pressure ratios. The results were compare to international literature an to simulation results. It has been foun out that uner the critical pressure ratio the qualitative agreements between the measurements an the calculations are quite goo, even quantitatively for the 2 mm orifice. It shows that, contrary to the Perry moel, the flow coefficient values almost remain constant in this region. There are significant ifferences between the moels an the measurements over the critical pressure ratio though, which are subject for future investigations. The installe measurement system is the part of an inustrial project of the Department of Flui Mechanics (KNORR-BREMSE). In the future the test bench will play an important role in the life of the epartment. It will be use in the eucation (Avance flow measurements) an also in the research an evelopment work. ACKNOWLEDGEMENT This work has been supporte by the Hungarian National Fun for Science an Research uner contract No. OTKA T

8 REFERENCES [1] Dr. Szőcs K., Kőfalusi P., Németh S., 1997, Brake systems (in Hungarian), Maróti-Goai Publishing [2] Josef M., 1996, ABS-TCS-VDC Where Will the Technology Lea Us?, Sale international [3] Dr. Varga F., Dr. Szőcs K., Juhász A., 1998, Knorr-Bremse ABS-ASR, Knorr-Bremse [4] Szente V., Va J., Lóránt G., Fries A., 2001, Computational an Experimental Investigation on Dynamics of Electric Braking Systems, Proc. 7th Scaninavian International Conference on Flui Power, Vol. 1, pp [5] Bieaux E., Scavara S., 1998, A Pneumatic Library for AMESim, Proc. ASME'98 Conference, Anaheim, California. [6] Perry J. A., 1949, Critical flow through sharpege orifices, Trans. ASME, Vol. 71. [7] Blaine W. A., 1967, The analysis an esign of pneumatic systems, John Wiley & Sons [8] McCloy D., Martin H. R., 1980, Control of Flui Power: Analysis an esign, Chichester, Ellis Horwoo [9] AMESim on-line ocumentation v4.1 (Mechanical / Signal, Control an Observers / Pneumatic libraries), 2003, [10] Grace H. P., Lapple C. E., 1951, Discharge coefficients for small-iameter orifices an flow nozzles, Trans. ASME, Vol. 73, pp [11] Jobson D. A., 1955, On the flow of a compressible flui through orifices, Proc. IME, Vol. 169, pp [12] Tsai D. H., Cassiy E. C., 1961, Dynamic behavior of simple pneumatic pressure reucer, J. Basic Eng., Vol. 83. [13] ISO 6358:1989, Pneumatic flui power. Components using compressible fluis. Determination of flow rate characteristics [14] Jakub E., 1968, Electromagnets (in Hungarian), Hungarian Technical Publishing [15] Szente V., Va J., 2002, Experimental an Computational Investigation on Flow Characteristics of Solenoi Valves, Gépészet 2002 conference, Vol. 1., pp [16] Szente V., Va J., 2003, Computational an experimental investigation on the flow characteristics of small-scale pneumatic solenoi valves, 2n International Conference on Heat Transfer, Flui Mechanics an Thermoynamics, Victoria Falls, Zambia [17] FLUENT ocumentation v6.0.12, 2001,